(1) Field of the invention
The present invention relates generally to a system and method for automatically controlling a vehicle's speed to a desired cruising speed using a fuzzy estimation.
(2) Background of the art
A Japanese Patent Application First Publication (Tokkai) sho No. 60-4428 published on Jan. 10, 1985 and Japanese Utility Model Application First Publication (Zikkai) sho No. 61-39636 published on Mar. 13, 1986 exemplify systems for automatically controlling vehicle speeds to desired cruising speeds in which a vehicle speed is detected by means of a vehicle speed sensor, the detected vehicle speed is compared with a target vehicle speed set on the basis of a driver's decision, deviations as the result of comparison (a control deviation, first-order difference and second-order difference) are calculated, a controlled variable is set by substituting the values of deviations into a control calculation equation, an actuator for an engine throttle valve is activated on the basis of the result of calculation equation to adjust an opening angle of an engine throttle valve, so that the vehicular engine output is adjusted to cruise the vehicle at the desired target speed.
In addition, the vehicle speed is once stored into a memory at a time when the set cruising speed run by the above-described system is released. Then, when the driver operates to resume the cruise speed run by the system, a resume operation of the system causes the vehicle to run at the stored vehicle speed.
Furthermore, a method has been proposed to provide more precise cruise control over the vehicle speed by Professor Murakami of Kyushu Kogyo University such that a fuzzy estimation is applied to the above-described system in which the controlled variable is set according to respective membership functions corresponding to the control deviation, first-order difference, and second-order difference.
However, in the above-identified Japanese Patent and Utility Model application publications, the automatic vehicle cruising speed controlling systems set the controlled variable only on the basis of the arithmetically operated values derived from the vehicle speed.
In addition, in the above-described proposed method of the fuzzy estimation, a configuration (e.g., gradient) of the membership function remains the same irrespective of a running situation under actual control over the vehicle speed in the fuzzy estimation. Therefore, control accuracy is reduced when the vehicle runs on a descending or ascending slope in which control gain of the vehicle cruise controlling system needs to be increased and, on the other hand, a magnitude of overshooting of the controlled variable (an angular displacement per time between the instantaneous angular position and previous angular position of the engine throttle valve) becomes large during a light load of the engine if the gain of control is increased. At this time, comfortability of the vehicle consequently worsens.